"24" what? The 3/4" is in inches so multiplying those numbers together gives you units of "square feet inches". What does that mean? Do you understand what those numbers mean? First, the 3/4" is the thickness of the plywood and has nothing to do with the area it will cover. One sheet of plywood 4' by 8' (assuming those measurements are actual and not "nominal") has area 4 x 8= 32 square feet. The floor you want to cover has area 24 x 32= 768 square feet. How many times will 32 divide into 768? (Actually, I find the answer rather amusing!)